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"This would see people being given warnings rather than being issued with fines," police said.

Smoking advocates called the new law unenforceable, but health officials and anti-smoking groups hailed the measure as the most important since a general ban on smoking in workplaces and enclosed public spaces took effect in England in 2007.

"Today is truly a cause for celebration for all those who care about protecting the health of generations to come," said Penny Woods, chief of the British Lung Foundation.

Public health officials say second-hand smoke can cause a range of health problems. "Among very small children, it's linked to sudden infant death syndrome, and as the child gets older it impairs development of the lungs, so asthma is more common," Hazel Cheeseman, of the group Action on Smoking and Health, told CBS News.

The law was adopted on the heels of an experiment that showed second-hand smoke in a car reached levels 100 times above safety guidelines.

The ban means anyone who smokes inside a car with children under 18 inside will face a 50 pounds ($75) fine. Drivers face the same penalty even if they are not the one smoking.

The law applies even if vehicle windows or a sunroof are open. It does not apply to a convertible if the top is down and properly stowed - and use of electronic cigarettes is not affected by the legislation.

On the streets of London, the measure appeared to have widespread support. "You wouldn't sit in a baby's bedroom and smoke, would you?" one man said. Another woman agreed even though she's a smoker herself. "It's just unhealthy for the other passengers who don't smoke, and it's going through their lungs as well," she said.

In the U.S., a handful of states have enacted such bans. The activist group Americans for Nonsmokers' Rights says seven states have laws prohibiting smoking in cars when children are present. The age limits range from children 8 and under in Vermont to those 18 and under in California and Oregon.

Some lawmakers in New Jersey, Ohio and Michigan have recently proposed similar legislation.